Seata中AT模式的实现原理03-二阶段提交

全局事务提交

TM提交全局事务

当业务正常处理完毕后 本地事务全部提交完成，TM会将xid提交给TC，TC会返回当前事务状态，status由TC决定，TM最后会将xid从RootContext中解绑，全局事务结束。
TransactionalTemplate

    private void commitTransaction(GlobalTransaction tx) throws TransactionalExecutor.ExecutionException {
        try {
            triggerBeforeCommit();
            tx.commit();
            triggerAfterCommit();
        } catch (TransactionException txe) {
            // 4.1 Failed to commit
            throw new TransactionalExecutor.ExecutionException(tx, txe,
                TransactionalExecutor.Code.CommitFailure);
        }
    }

    @Override
    public void commit() throws TransactionException {
        if (role == GlobalTransactionRole.Participant) {
            // Participant has no responsibility of committing
            if (LOGGER.isDebugEnabled()) {
                LOGGER.debug("Ignore Commit(): just involved in global transaction [{}]", xid);
            }
            return;
        }
        assertXIDNotNull();
          // 默认重试5次 client.tm.commitRetryCount
        int retry = COMMIT_RETRY_COUNT <= 0 ? DEFAULT_TM_COMMIT_RETRY_COUNT : COMMIT_RETRY_COUNT;
        try {
            while (retry > 0) {
                try {
                // GlobalCommitRequest 发送全局事务id给TC，TC会返回全局事务状态给TM
                    status = transactionManager.commit(xid);
                    break;
                } catch (Throwable ex) {
                    LOGGER.error("Failed to report global commit [{}],Retry Countdown: {}, reason: {}", this.getXid(), retry, ex.getMessage());
                    retry--;
                    if (retry == 0) {
                        throw new TransactionException("Failed to report global commit", ex);
                    }
                }
            }
        } finally {
         // 将xid从ThreadLocal中移除，代表这个全局事务已经结束
            if (xid.equals(RootContext.getXID())) {
                suspend();
            }
        }
        if (LOGGER.isInfoEnabled()) {
            LOGGER.info("[{}] commit status: {}", xid, status);
        }
    }

TC处理全局事务提交请求

AbstractTCInboundHandler

    @Override
    public GlobalCommitResponse handle(GlobalCommitRequest request, final RpcContext rpcContext) {
        GlobalCommitResponse response = new GlobalCommitResponse();
        response.setGlobalStatus(GlobalStatus.Committing);
        exceptionHandleTemplate(new AbstractCallback<GlobalCommitRequest, GlobalCommitResponse>() {
            @Override
            public void execute(GlobalCommitRequest request, GlobalCommitResponse response)
                throws TransactionException {
                try {
                     //全局事务提交
                    doGlobalCommit(request, response, rpcContext);
                } catch (StoreException e) {
                    throw new TransactionException(TransactionExceptionCode.FailedStore,
                        String.format("global commit request failed. xid=%s, msg=%s", request.getXid(), e.getMessage()),
                        e);
                }
            }
           @Override
        public void onTransactionException(GlobalCommitRequest request, GlobalCommitResponse response,
                                           TransactionException tex) {
            super.onTransactionException(request, response, tex);
            // 设置当前全局事务状态
            checkTransactionStatus(request, response);
        }

        @Override
        public void onException(GlobalCommitRequest request, GlobalCommitResponse response, Exception rex) {
            // 设置当前全局事务状态
            super.onException(request, response, rex);
            checkTransactionStatus(request, response);
        }
        }, request, response);
        return response;
    }

DefaultCoordinator

    @Override
    protected void doGlobalCommit(GlobalCommitRequest request, GlobalCommitResponse response, RpcContext rpcContext)
            throws TransactionException {
        MDC.put(RootContext.MDC_KEY_XID, request.getXid());
        response.setGlobalStatus(core.commit(request.getXid()));
    }

public GlobalStatus commit(String xid) throws TransactionException {
    GlobalSession globalSession = SessionHolder.findGlobalSession(xid);
    if (globalSession == null) {
        return GlobalStatus.Finished;
    }
    globalSession.addSessionLifecycleListener(SessionHolder.getRootSessionManager());
    boolean shouldCommit = SessionHolder.lockAndExecute(globalSession, () -> {
        if (globalSession.getStatus() == GlobalStatus.Begin) {
            // 如果分支事务都是AT模式，释放全局锁，delete from lock_table where xid = ?
            globalSession.closeAndClean();
            // 如果分支事务都是AT模式，或分支事务有一阶段提交失败的，则可以执行异步提交
            if (globalSession.canBeCommittedAsync()) {
                // 执行异步提交，更新全局事务状态为AsyncCommitting，update global_table set status = AsyncCommitting where xid = ?
                globalSession.asyncCommit();
                MetricsPublisher.postSessionDoneEvent(globalSession, GlobalStatus.Committed, false, false);
                return false;
            } else {
                globalSession.changeGlobalStatus(GlobalStatus.Committing);
                return true;
            }
        }
        return false;
    });

    if (shouldCommit) { // 同步提交
        boolean success = doGlobalCommit(globalSession, false);
        // ...
    } else { // 异步提交
        return globalSession.getStatus() == GlobalStatus.AsyncCommitting ? GlobalStatus.Committed : globalSession.getStatus();
    }
}

DefaultCoordinator在初始化时启动了一个1秒执行一次的定时任务，通过distributed_lock表获取分布式锁，同一时间只有一个TC能执行异步提交任务。
DefaultCoordinator

public void init() {
    // 全局事务二阶段异步提交
    asyncCommitting.scheduleAtFixedRate(
        () -> SessionHolder.distributedLockAndExecute(ASYNC_COMMITTING, this::handleAsyncCommitting), 0,
        1000, TimeUnit.MILLISECONDS);
}

    protected void handleAsyncCommitting() {
        SessionCondition sessionCondition = new SessionCondition(GlobalStatus.AsyncCommitting);
        Collection<GlobalSession> asyncCommittingSessions =
                SessionHolder.getAsyncCommittingSessionManager().findGlobalSessions(sessionCondition);
        if (CollectionUtils.isEmpty(asyncCommittingSessions)) {
            return;
        }
        SessionHelper.forEach(asyncCommittingSessions, asyncCommittingSession -> {
            try {
                asyncCommittingSession.addSessionLifecycleListener(SessionHolder.getRootSessionManager());
                //提交
                core.doGlobalCommit(asyncCommittingSession, true);
            } catch (TransactionException ex) {
                LOGGER.error("Failed to async committing [{}] {} {}", asyncCommittingSession.getXid(), ex.getCode(), ex.getMessage(), ex);
            }
        });
    }

DefaultCore.doGlobalCommit
1.循环处理每个分支事务，发送BranchCommitRequest给RM，RM删除undo_log；
2.如果RM处理二阶段提交成功，返回分支事务状态PhaseTwo_Committed，TC删除branch_table中对应的分支事务；
3.如果所有分支事务都处理成功，TC删除global_table中的全局事务；

    @Override
    public boolean doGlobalCommit(GlobalSession globalSession, boolean retrying) throws TransactionException {
        boolean success = true;
        // start committing event
        MetricsPublisher.postSessionDoingEvent(globalSession, retrying);
        //如果是saga模式  
        if (globalSession.isSaga()) {
            success = getCore(BranchType.SAGA).doGlobalCommit(globalSession, retrying);
        } else {
            //循环分支事务
            Boolean result = SessionHelper.forEach(globalSession.getSortedBranches(), branchSession -> {
                // if not retrying, skip the canBeCommittedAsync branches
                if (!retrying && branchSession.canBeCommittedAsync()) {
                    return CONTINUE;
                }

                BranchStatus currentStatus = branchSession.getStatus();
                if (currentStatus == BranchStatus.PhaseOne_Failed) {
                    SessionHelper.removeBranch(globalSession, branchSession, !retrying);
                    return CONTINUE;
                }
                try {
                    //发送BranchCommitRequest给RM，RM会删除undo_log
                    BranchStatus branchStatus = getCore(branchSession.getBranchType()).branchCommit(globalSession, branchSession);
                    if (isXaerNotaTimeout(globalSession,branchStatus)) {
                        LOGGER.info("Commit branch XAER_NOTA retry timeout, xid = {} branchId = {}", globalSession.getXid(), branchSession.getBranchId());
                        branchStatus = BranchStatus.PhaseTwo_Committed;
                    }
                    switch (branchStatus) {
                        case PhaseTwo_Committed:
                        //删除branch_table中的分支事务记录
                            SessionHelper.removeBranch(globalSession, branchSession, !retrying);
                            return CONTINUE;
                        case PhaseTwo_CommitFailed_Unretryable:
                            //not at branch
                            SessionHelper.endCommitFailed(globalSession, retrying);
                            LOGGER.error("Committing global transaction[{}] finally failed, caused by branch transaction[{}] commit failed.", globalSession.getXid(), branchSession.getBranchId());
                            return false;

                        default:
                            if (!retrying) {
                                globalSession.queueToRetryCommit();
                                return false;
                            }
                            if (globalSession.canBeCommittedAsync()) {
                                LOGGER.error("Committing branch transaction[{}], status:{} and will retry later",
                                    branchSession.getBranchId(), branchStatus);
                                return CONTINUE;
                            } else {
                                LOGGER.error(
                                    "Committing global transaction[{}] failed, caused by branch transaction[{}] commit failed, will retry later.", globalSession.getXid(), branchSession.getBranchId());
                                return false;
                            }
                    }
                } catch (Exception ex) {
                    StackTraceLogger.error(LOGGER, ex, "Committing branch transaction exception: {}",
                        new String[] {branchSession.toString()});
                    if (!retrying) {
                        globalSession.queueToRetryCommit();
                        throw new TransactionException(ex);
                    }
                }
                //某个分支事务处理失败，继续处理后续分支事务
                return CONTINUE;
            });
            // Return if the result is not null
            if (result != null) {
                return result;
            }
            //If has branch and not all remaining branches can be committed asynchronously,
            //do print log and return false
            if (globalSession.hasBranch() && !globalSession.canBeCommittedAsync()) {
                LOGGER.info("Committing global transaction is NOT done, xid = {}.", globalSession.getXid());
                return false;
            }
            if (!retrying) {
                //contains not AT branch
                globalSession.setStatus(GlobalStatus.Committed);
            }
        }
        // if it succeeds and there is no branch, retrying=true is the asynchronous state when retrying. EndCommitted is
        // executed to improve concurrency performance, and the global transaction ends..
        if (success && globalSession.getBranchSessions().isEmpty()) {
        //如果所有分支事务被删除，则删除全局事务 delete from global_table where xid = ?
            SessionHelper.endCommitted(globalSession, retrying);
            LOGGER.info("Committing global transaction is successfully done, xid = {}.", globalSession.getXid());
        }
        return success;
    }

RM处理分支事务的提交

AbstractRMHandler

    @Override
    public BranchCommitResponse handle(BranchCommitRequest request) {
        BranchCommitResponse response = new BranchCommitResponse();
        exceptionHandleTemplate(new AbstractCallback<BranchCommitRequest, BranchCommitResponse>() {
            @Override
            public void execute(BranchCommitRequest request, BranchCommitResponse response)
                throws TransactionException {
                doBranchCommit(request, response);
            }
        }, request, response);
        return response;
    }

AbstractRMHandler实际委托ResourceManager实现分支事务提交。
AsyncWorker异步处理分支事务提交，将xid、branchId、resourceId封装为Phase2Context，放入阻塞队列，等待一个1秒执行一次的定时任务消费队列中的Phase2Context。

// DataSourceManager
private final AsyncWorker asyncWorker = new AsyncWorker(this);
public BranchStatus branchCommit(BranchType branchType, String xid, long branchId, String resourceId,
                                 String applicationData) throws TransactionException {
    return asyncWorker.branchCommit(xid, branchId, resourceId);
}

    private void addToCommitQueue(Phase2Context context) {
        if (commitQueue.offer(context)) {
            return;
        }
        CompletableFuture.runAsync(this::doBranchCommitSafely, scheduledExecutor)
                .thenRun(() -> addToCommitQueue(context));
    }

最后会由AsyncWorker执行 从阻塞队列里面poll数据 然后执行 实际上就是删除 undo_log数据

private void deleteUndoLog(final Connection conn, UndoLogManager undoLogManager, List<Phase2Context> contexts) {
    Set<String> xids = new LinkedHashSet<>(contexts.size());
    Set<Long> branchIds = new LinkedHashSet<>(contexts.size());
    contexts.forEach(context -> {
        xids.add(context.xid);
        branchIds.add(context.branchId);
    });

    try {
        undoLogManager.batchDeleteUndoLog(xids, branchIds, conn);
        if (!conn.getAutoCommit()) {
            conn.commit();
        }
    } catch (SQLException e) {
        LOGGER.error("Failed to batch delete undo log", e);
        try {
            conn.rollback();
            addAllToCommitQueue(contexts);
        } catch (SQLException rollbackEx) {
            LOGGER.error("Failed to rollback JDBC resource after deleting undo log failed", rollbackEx);
        }
    }
}

总结

1.当一阶段提交没有任何异常情况下进行二阶段提交 TM会发送提交全局事务请求给TC
2.TC接收到全局事务提交请求之后会循环所有的分支事务 往RM发送分支事务提交的请求
3.RM接收到分支事务提交请求之后会委派给AsyncWorker去处理 从阻塞队列里面拿数据
4.AsyncWorker处理的时候其实就是将undo_log的数据删除
5.当RM处理完之后返回TC 然后会将全局锁删除 就是 lock_table表数据
6.将分支事务状态变为二阶段已提交
7.然后会将分支事务表brach_table数据给清空
8.然后会将global_table数据删除
9.当TC端所有处理完成之后 TM收到响应信息把XID给解绑

TM全局事务回滚

TM处理全局事务回滚向TC发起请求

当一阶段提交出问题 会触发TM的回滚 然后交给TC执行

    private void completeTransactionAfterThrowing(TransactionInfo txInfo, GlobalTransaction tx, Throwable originalException) throws TransactionalExecutor.ExecutionException {
        //roll back
        if (txInfo != null && txInfo.rollbackOn(originalException)) {
            try {
               // 默认所有异常都会回滚
                rollbackTransaction(tx, originalException);
            } catch (TransactionException txe) {
                // Failed to rollback
                throw new TransactionalExecutor.ExecutionException(tx, txe,
                        TransactionalExecutor.Code.RollbackFailure, originalException);
            }
        } else {
         // 如果GlobalTransactional排除部分异常，可以选择提交
            // not roll back on this exception, so commit
            commitTransaction(tx);
        }
    }

    private void rollbackTransaction(GlobalTransaction tx, Throwable originalException) throws TransactionException, TransactionalExecutor.ExecutionException {
        triggerBeforeRollback();
        tx.rollback();
        triggerAfterRollback();
        // 3.1 Successfully rolled back
        throw new TransactionalExecutor.ExecutionException(tx, GlobalStatus.RollbackRetrying.equals(tx.getLocalStatus())
            ? TransactionalExecutor.Code.RollbackRetrying : TransactionalExecutor.Code.RollbackDone, originalException);
    }

TM向TC发送回滚请求，请求包含xid，由TC返回全局事务状态。这和TM发送提交请求类似。

    @Override
    public void rollback() throws TransactionException {
        if (role == GlobalTransactionRole.Participant) {
            // Participant has no responsibility of rollback
            if (LOGGER.isDebugEnabled()) {
                LOGGER.debug("Ignore Rollback(): just involved in global transaction [{}]", xid);
            }
            return;
        }
        assertXIDNotNull();
// 默认重试5次 client.tm.rollbackRetryCount
        int retry = ROLLBACK_RETRY_COUNT <= 0 ? DEFAULT_TM_ROLLBACK_RETRY_COUNT : ROLLBACK_RETRY_COUNT;
        try {
            while (retry > 0) {
                try {
                  // GlobalRollbackRequest，包含全局事务id给TC，TC会返回全局事务状态给TM
                    status = transactionManager.rollback(xid);
                    break;
                } catch (Throwable ex) {
                    LOGGER.error("Failed to report global rollback [{}],Retry Countdown: {}, reason: {}", this.getXid(), retry, ex.getMessage());
                    retry--;
                    if (retry == 0) {
                        throw new TransactionException("Failed to report global rollback", ex);
                    }
                }
            }
        } finally {
            //解绑全局事务ID
            if (xid.equals(RootContext.getXID())) {
                suspend();
            }
        }
        if (LOGGER.isInfoEnabled()) {
            LOGGER.info("[{}] rollback status: {}", xid, status);
        }
    }

TC处理全局事务回滚

DefaultCore

public GlobalStatus rollback(String xid) throws TransactionException {
    GlobalSession globalSession = SessionHolder.findGlobalSession(xid);
    if (globalSession == null) {
        return GlobalStatus.Finished;
    }
    globalSession.addSessionLifecycleListener(SessionHolder.getRootSessionManager());
    boolean shouldRollBack = SessionHolder.lockAndExecute(globalSession, () -> {
        globalSession.close();
        if (globalSession.getStatus() == GlobalStatus.Begin) {
            // 将全局锁lock_table状态更新为Rollbacking 
            // 将全局事务global_table状态更新为Rollbacking
            globalSession.changeGlobalStatus(GlobalStatus.Rollbacking);
            return true;
        }
        return false;
    });
    if (!shouldRollBack) {
        return globalSession.getStatus();
    }
    // 执行全局回滚
    boolean rollbackSuccess = doGlobalRollback(globalSession, false);
    return rollbackSuccess ? GlobalStatus.Rollbacked : globalSession.getStatus();
}

在变更完中间状态后，执行doGlobalRollback方法实际执行二阶段回滚
二阶段回滚逻辑：
1：TC循环向所有RM发送BranchRollbackRequest；
2：如果RM返回PhaseTwo_Rollbacked，则删除对应分支事务；发生异常或返回状态非回滚成功，将全局事务标记为RollbackRetrying，等待后续补偿执行全局回滚；
3：如果所有RM二阶段回滚成功，对于file存储模式，直接删除全局事务（因为file模式在分支注册和二阶段回滚操作上都加了锁）；对于db/redis存储模式，需要异步再次执行doGlobalRollback，确保不会有分支事务注册与二阶段回滚同时发生，造成分支事务注册成功，二阶段回滚没有清理干净全局锁和分支事务；
DefaultCore

public boolean doGlobalRollback(GlobalSession globalSession, boolean retrying) throws TransactionException {
    boolean success = true;
    //循环向RM发送回滚分支事务请求
    Boolean result = SessionHelper.forEach(globalSession.getReverseSortedBranches(), branchSession -> {
        BranchStatus currentBranchStatus = branchSession.getStatus();
        if (currentBranchStatus == BranchStatus.PhaseOne_Failed) {
            SessionHelper.removeBranch(globalSession, branchSession, !retrying);
            return CONTINUE;
        }
        try {
            //  发送BranchRollbackRequest
            BranchStatus branchStatus = branchRollback(globalSession, branchSession);
            switch (branchStatus) {
                case PhaseTwo_Rollbacked:
                    // 释放全局锁，删除分支事务
                    SessionHelper.removeBranch(globalSession, branchSession, !retrying);
                    return CONTINUE;
                case PhaseTwo_RollbackFailed_Unretryable: // 回滚失败且无法重试成功
                    SessionHelper.endRollbackFailed(globalSession, retrying);
                    return false;
                default:
                    // 如果RM回滚失败 全局事务状态变为RollbackRetrying 等待重试
                    if (!retrying) {
                        globalSession.queueToRetryRollback();
                    }
                    return false;
            }
        } catch (Exception ex) {
            if (!retrying) {
                // 如果步骤异常 全局事务状态变为RollbackRetrying 等待重试
                globalSession.queueToRetryRollback();
            }
            throw new TransactionException(ex);
        }
    });
    // 如果存在一个分支事务回滚失败，则返回false
    if (result != null) {
        return result;
    }

    // 对于file模式，直接删除全局事务
    // 对于db/redis模式，异步再次执行doGlobalRollback，这里不做任何处理
    //  防止由于各种网络波动造成分支事务注册成功lock_table和branch_table中始终有残留数据
    //  导致全局锁一直被占用，无法释放
    if (success) {
        SessionHelper.endRollbacked(globalSession, retrying);
    }
    return success;
}

// SessionHelper
public static void endRollbacked(GlobalSession globalSession, boolean retryGlobal) throws TransactionException {
     // 如果是重试 或 file模式
     if (retryGlobal || !DELAY_HANDLE_SESSION) {
        long beginTime = System.currentTimeMillis();
        GlobalStatus currentStatus = globalSession.getStatus();
        boolean retryBranch =
            currentStatus == GlobalStatus.TimeoutRollbackRetrying || currentStatus == GlobalStatus.RollbackRetrying;
        if (isTimeoutGlobalStatus(currentStatus)) {
            globalSession.changeGlobalStatus(GlobalStatus.TimeoutRollbacked);
        } else {
            globalSession.changeGlobalStatus(GlobalStatus.Rollbacked);
        }
        // 删除全局事务global_table
        globalSession.end();
    }
}

RM处理分支事务回滚

AbstractRMHandler

    @Override
    public BranchRollbackResponse handle(BranchRollbackRequest request) {
        BranchRollbackResponse response = new BranchRollbackResponse();
        exceptionHandleTemplate(new AbstractCallback<BranchRollbackRequest, BranchRollbackResponse>() {
            @Override
            public void execute(BranchRollbackRequest request, BranchRollbackResponse response)
                throws TransactionException {
                //分支事务回滚
                doBranchRollback(request, response);
            }
        }, request, response);
        return response;
    }
  protected void doBranchRollback(BranchRollbackRequest request, BranchRollbackResponse response)
    throws TransactionException {
    String xid = request.getXid();
    long branchId = request.getBranchId();
    String resourceId = request.getResourceId();
    String applicationData = request.getApplicationData();
    BranchStatus status = getResourceManager().branchRollback(request.getBranchType(), xid, branchId, resourceId,
        applicationData);
    response.setXid(xid);
    response.setBranchId(branchId);
    response.setBranchStatus(status);
}

DataSourceManager

public BranchStatus branchRollback(BranchType branchType, String xid, long branchId, String resourceId,
                                   String applicationData) throws TransactionException {
    DataSourceProxy dataSourceProxy = get(resourceId);
    if (dataSourceProxy == null) {
        throw new ShouldNeverHappenException(String.format("resource: %s not found",resourceId));
    }
    try {
        UndoLogManagerFactory.getUndoLogManager(dataSourceProxy.getDbType()).undo(dataSourceProxy, xid, branchId);
    } catch (TransactionException te) {
        if (te.getCode() == TransactionExceptionCode.BranchRollbackFailed_Unretriable) {
            return BranchStatus.PhaseTwo_RollbackFailed_Unretryable;
        } else {
            return BranchStatus.PhaseTwo_RollbackFailed_Retryable;
        }
    }
    return BranchStatus.PhaseTwo_Rollbacked;

}

RM二阶段回滚就是执行undoLog里的前置镜像，回滚一阶段的本地事务。具体逻辑都在

// AbstractUndoLogManager
public void undo(DataSourceProxy dataSourceProxy, String xid, long branchId) throws TransactionException {
    Connection conn = null;
    ResultSet rs = null;
    PreparedStatement selectPST = null;
    boolean originalAutoCommit = true;
    for (; ; ) {
        try {
            conn = dataSourceProxy.getPlainConnection();
            if (originalAutoCommit = conn.getAutoCommit()) {
                conn.setAutoCommit(false);
            }
            // Step1 根据xid+branchId查询undo_log
            selectPST = conn.prepareStatement(SELECT_UNDO_LOG_SQL);
            selectPST.setLong(1, branchId);
            selectPST.setString(2, xid);
            rs = selectPST.executeQuery();

            boolean exists = false;
            while (rs.next()) {
                exists = true;
                int state = rs.getInt(ClientTableColumnsName.UNDO_LOG_LOG_STATUS);
                if (!canUndo(state)) {
                    return;
                }

                String contextString = rs.getString(ClientTableColumnsName.UNDO_LOG_CONTEXT);
                Map<String, String> context = parseContext(contextString);
                byte[] rollbackInfo = getRollbackInfo(rs);

                String serializer = context == null ? null : context.get(UndoLogConstants.SERIALIZER_KEY);
                UndoLogParser parser = serializer == null ? UndoLogParserFactory.getInstance()
                    : UndoLogParserFactory.getInstance(serializer);
                BranchUndoLog branchUndoLog = parser.decode(rollbackInfo);

                try {
                    setCurrentSerializer(parser.getName());
                    List<SQLUndoLog> sqlUndoLogs = branchUndoLog.getSqlUndoLogs();
                    if (sqlUndoLogs.size() > 1) {
                        Collections.reverse(sqlUndoLogs);
                    }
                    // Step2 回滚本地事务
                    for (SQLUndoLog sqlUndoLog : sqlUndoLogs) {
                        TableMeta tableMeta = TableMetaCacheFactory.getTableMetaCache(dataSourceProxy.getDbType()).getTableMeta(
                            conn, sqlUndoLog.getTableName(), dataSourceProxy.getResourceId());
                        sqlUndoLog.setTableMeta(tableMeta);
                        AbstractUndoExecutor undoExecutor = UndoExecutorFactory.getUndoExecutor(
                            dataSourceProxy.getDbType(), sqlUndoLog);
                        undoExecutor.executeOn(conn);
                    }
                } finally {
                    removeCurrentSerializer();
                }
            }
            // See https://github.com/seata/seata/issues/489
            // Step3 undo_log处理
            if (exists) {
                // RM一阶段正常提交，删除undoLog
                deleteUndoLog(xid, branchId, conn);
                conn.commit();
            } else {
                // RM一阶段提交失败，TC由于全局事务超时发起回滚
                // 此时RM的本地事务可能提交，这里插入一个undo_log来防止RM提交事务成功导致数据不一致
                insertUndoLogWithGlobalFinished(xid, branchId, UndoLogParserFactory.getInstance(), conn);
                conn.commit();
            }

            return;
        } catch (SQLIntegrityConstraintViolationException e) {
            // insertUndoLogWithGlobalFinished 如果插入undo_log由于唯一约束失败，那么执行重试回滚
            // 日志...
        } catch (Throwable e) {
            if (conn != null) {
                try {
                    conn.rollback();
                } catch (SQLException rollbackEx) {
                    LOGGER.warn("Failed to close JDBC resource while undo ... ", rollbackEx);
                }
            }
            throw new BranchTransactionException(BranchRollbackFailed_Retriable, String
                .format("Branch session rollback failed and try again later xid = %s branchId = %s %s", xid,
                    branchId, e.getMessage()), e);

        } finally {
            // 关闭资源
        }
    }
}

二阶段回滚RM本地事务分为三步：
1.查询分支事务对应undoLog，select from undo_log where xid = ? and branch_id = ? for update；
2.AbstractUndoExecutor利用undoLog中的前置镜像回滚本地事务；
3.处理undoLog；

回滚一阶段本地事务

根据undoLog回滚一阶段本地事务。

// AbstractUndoExecutor
public void executeOn(Connection conn) throws SQLException {
    // Step1 比较当前数据和前后镜像，如果情况允许才会回滚
    if (IS_UNDO_DATA_VALIDATION_ENABLE && !dataValidationAndGoOn(conn)) {
        return;
    }
    PreparedStatement undoPST = null;
    try {
        // Step2 构建回滚sql 如 UPDATE a SET x = ? WHERE pk1 in (?)
        String undoSQL = buildUndoSQL();
        undoPST = conn.prepareStatement(undoSQL);
        TableRecords undoRows = getUndoRows();
        // 根据主键回滚前置镜像中每一行数据
        for (Row undoRow : undoRows.getRows()) {
            ArrayList<Field> undoValues = new ArrayList<>();
            List<Field> pkValueList = getOrderedPkList(undoRows, undoRow, getDbType(conn));
            for (Field field : undoRow.getFields()) {
                if (field.getKeyType() != KeyType.PRIMARY_KEY) {
                    undoValues.add(field);
                }
            }
            // Step3 替换占位符
            undoPrepare(undoPST, undoValues, pkValueList);
            // Step4 根据id回滚一行
            undoPST.executeUpdate();
        }

    } catch (Exception ex) {
        if (ex instanceof SQLException) {
            throw (SQLException) ex;
        } else {
            throw new SQLException(ex);
        }
    }
    finally {
        //important for oracle
        IOUtil.close(undoPST);
    }

}

在执行回滚之前，dataValidationAndGoOn方法会比较当前数据和前后镜像，如果情况允许才会回滚

protected boolean dataValidationAndGoOn(Connection conn) throws SQLException {
    TableRecords beforeRecords = sqlUndoLog.getBeforeImage();
    TableRecords afterRecords = sqlUndoLog.getAfterImage();
    // case1 前后镜像一致，不需要回滚，返回false
    Result<Boolean> beforeEqualsAfterResult = DataCompareUtils.isRecordsEquals(beforeRecords, afterRecords);
    if (beforeEqualsAfterResult.getResult()) {
        return false;
    }

    // select for update by 主键 获取当前快照数据
    TableRecords currentRecords = queryCurrentRecords(conn);
    // case2 当前快照数据和后置镜像不一致
    Result<Boolean> afterEqualsCurrentResult = DataCompareUtils.isRecordsEquals(afterRecords, currentRecords);
    if (!afterEqualsCurrentResult.getResult()) {
        // case2-1 如果前置镜像与当前快照一致，也不需要回滚，返回false
        Result<Boolean> beforeEqualsCurrentResult = DataCompareUtils.isRecordsEquals(beforeRecords, currentRecords);
        if (beforeEqualsCurrentResult.getResult()) {
            return false;
        } else {
            // case2-2 当前快照数据和后置镜像不一致，抛出异常，有脏数据
            throw new SQLException("Has dirty records when undo.");
        }
    }
    // case3 当前快照数据与后置镜像一致，可以执行回滚
    return true;
}

处理undolog

如果RM一阶段commit成功，undo_log存在，即exists=true，只要删除undo_log即可，与回滚事务一起提交；
如果RM一阶段未commit成功，undo_log不存在，可能由于RM一阶段业务处理超时，导致TC判断全局事务超时发起二阶段回滚，此时RM一阶段事务提交和RM二阶段回滚可能同时发生。
为了确保数据一致性，利用undo_log的xid和branch_id的唯一约束，插入一条status=GlobalFinished的数据来解决这个问题。
如果RM一阶段事务提交成功，即一阶段提交插入status=Normal的undo_log成功，那么在二阶段回滚时insertUndoLogWithGlobalFinished会抛出SQLIntegrityConstraintViolationException唯一约束冲突，AbstractUndoLogManager.undo重试二阶段回滚本地事务，最终会回滚成功；
如果二阶段回滚插入status=GlobalFinished的undo_log成功，那么RM一阶段就不会提交成功（发生唯一约束冲突），从而解决数据一致性问题。
AbstractUndoLogManager

public void undo(DataSourceProxy dataSourceProxy, String xid, long branchId) throws TransactionException {
    for (; ; ) {
        try {
            // ...
            // See https://github.com/seata/seata/issues/489
            // Step3 undo_log处理
            if (exists) {
                // RM一阶段正常提交，删除undoLog
                deleteUndoLog(xid, branchId, conn);
                conn.commit();
            } else {
                // RM一阶段超时，TC由于全局事务超时发起回滚
                // 此时RM的本地事务可能同时提交，这里插入一个undo_log来防止RM提交事务成功导致数据不一致
                insertUndoLogWithGlobalFinished(xid, branchId, UndoLogParserFactory.getInstance(), conn);
                conn.commit();
            }

            return;
        } catch (SQLIntegrityConstraintViolationException e) {
            // insertUndoLogWithGlobalFinished 如果插入undo_log由于唯一约束失败，那么执行重试回滚
            // 日志...
        } 
        // ...
    }
}

// MySQLUndoLogManager
protected void insertUndoLogWithGlobalFinished(String xid, long branchId, UndoLogParser parser, Connection conn) throws SQLException {
    insertUndoLog(xid, branchId, buildContext(parser.getName(), CompressorType.NONE), parser.getDefaultContent(), State.GlobalFinished, conn);
}

总结

1.当一阶段业务或者其他发生异常的时候会触发二阶段的回滚
2.TM发送二阶段回滚请求给TC
3.TC接收回滚请求进行处理 会先遍历所有的分支事务 然后往RM去发送回滚的请求
4.RM接收请求之后会基于undolog去进行回滚
4.1会先查询当前undo_log 然后拿到前置和后置镜像 去比较前置和后置镜像和当前需要执行的语句是不是一致的
4.2如果后置镜像和需要回滚的语句是一样的那正常回滚 如果不一样则代表有脏数据的情况抛异常
5.如果一阶段commit undo_log存在 只要删除undo_log即可，与回滚事务一起提交
6.如果RM一阶段未commit成功，undo_log不存在，可能由于RM一阶段业务处理超时，导致TC判断全局事务超时发起二阶段回滚，此时RM一阶段事务提交和RM二阶段回滚可能同时发生。
7.当RM处理完之后响应TC TC会删除分支事务 然后把状态改为二阶段已回滚
8.当所有分支事务全部回滚结束之后 将全局事务删除

二阶段失败重试机制

DefaultCoordinator每秒执行handleRetryRollbacking方法，一方面是处理二阶段回滚的重试工作，另一方面处理正常二阶段回滚的剩余数据处理，比如残留的全局锁、分支事务，以及db/redis存储模式下需要异步清理的全局事务。
重试工作会一直进行下去，除非RM返回PhaseTwo_Rollbacked回滚成功，或PhaseTwo_RollbackFailed_Unretryable（代表回滚失败但是重试也不可能成功，属于不可恢复异常，目前这一状态只有XA模式下会返回，所以在AT模式下会重试到RM返回二阶段回滚成功为止）。

// DefaultCoordinator
protected void handleRetryRollbacking() {
    // 查询TimeoutRollbacking,TimeoutRollbackRetrying, RollbackRetrying, Rollbacking 的全局事务
    SessionCondition sessionCondition = new SessionCondition(rollbackingStatuses);
    sessionCondition.setLazyLoadBranch(true);
    Collection<GlobalSession> rollbackingSessions =
        SessionHolder.getRetryRollbackingSessionManager().findGlobalSessions(sessionCondition);
    if (CollectionUtils.isEmpty(rollbackingSessions)) {
        return;
    }
    long now = System.currentTimeMillis();
    SessionHelper.forEach(rollbackingSessions, rollbackingSession -> {
        try {
            // 如果是正在回滚的事务，需要等待开启全局事务的130秒后变为DeadSession再处理
            if (rollbackingSession.getStatus().equals(GlobalStatus.Rollbacking)
                && !rollbackingSession.isDeadSession()) {
                return;
            }
            // 默认MAX_ROLLBACK_RETRY_TIMEOUT=-1，不走这里
            if (isRetryTimeout(now, MAX_ROLLBACK_RETRY_TIMEOUT.toMillis(), rollbackingSession.getBeginTime())) {
                // ...
                return;
            }
            // 再次执行全局事务回滚逻辑
            rollbackingSession.addSessionLifecycleListener(SessionHolder.getRootSessionManager());
            core.doGlobalRollback(rollbackingSession, true);
        } catch (TransactionException ex) {
            LOGGER.info("Failed to retry rollbacking [{}] {} {}", rollbackingSession.getXid(), ex.getCode(), ex.getMessage());
        }
    });
}

全局事务超时处理

DefaultCoordinator每秒执行timeoutCheck方法，将处于begin状态的超时全局事务更新为TimeoutRollbacking状态，交由二阶段回滚重试定时任务（handleRetryRollbacking），异步执行回滚操作。

// DefaultCoordinator
protected void timeoutCheck() {
    // 1. 查询状态处于begin的全局事务
    SessionCondition sessionCondition = new SessionCondition(GlobalStatus.Begin);
    sessionCondition.setLazyLoadBranch(true);
    Collection<GlobalSession> beginGlobalsessions =
        SessionHolder.getRootSessionManager().findGlobalSessions(sessionCondition);
    if (CollectionUtils.isEmpty(beginGlobalsessions)) {
        return;
    }
    SessionHelper.forEach(beginGlobalsessions, globalSession -> {
        SessionHolder.lockAndExecute(globalSession, () -> {
            // 2. 校验是否超时
            if (globalSession.getStatus() != GlobalStatus.Begin || !globalSession.isTimeout()) {
                return false;
            }
            globalSession.addSessionLifecycleListener(SessionHolder.getRootSessionManager());
            globalSession.close();

            // 3. 更新全局事务状态为TimeoutRollbacking，通过handleRetryRollbacking异步处理二阶段回滚
            globalSession.setStatus(GlobalStatus.TimeoutRollbacking);

            globalSession.addSessionLifecycleListener(SessionHolder.getRetryRollbackingSessionManager());
            SessionHolder.getRetryRollbackingSessionManager().addGlobalSession(globalSession);
            return true;
        });
    });
}